A system for in situ replacement of cutting means for a ground drill is described in Applicant's International application no. PCT/AU94/00322 (WO 94/29567), the contents of which are incorporated herein by way of reference. The system in WO 94/29567 comprises a drive sub which is adapted for connection to a lower end of a core barrel attached to a drill pipe; a tool for installing and retracting drill bit segments from the drive sub; and, an insert or bit locking sleeve for selectively locking the bit segments into seats provided about the inner circumferential surface of an end of the drive sub and subsequently releasing the bit segments for those seats. The tool includes a main body portion and a sleeve slidably mounted thereon. Installation latch dogs provided in the tool extend from apertures or slots cut in the sleeve so as to engage the bit locking sleeve and force it into an installation position in which it locks the bit segments in a cutting position about the drive sub. The tool further includes retrieval latch dogs which can extend from different slots provided in the sleeve for engaging the bit locking sleeve and pulling it upwardly into a retrieval position in which the bit segments can be retrieved from the drive sub.
A slidable cradle extends from a lower end of the tool for carrying the bit segments to and from the drive sub. When installing the bit segments, the cradle is extended from the lower end or head of the tool against the bias of a spring. Bit segments are held by rubber bands about the cradle with one end abutting a stop provided at one of the cradle and an opposite end bearing against the head of the tool. When the tool is lowered into the ground drill (comprising the combination of the drill tube, core barrel and drive sub) and reaches a predetermined position within the drive sub (that being the point of engagement with the bit locking sleeve), the sleeve is caused to move relative to the main body of the tool which in turn releases a set of pins holding the spring about the cradle in compression. This fires the cradle so that the spring is able to expand, retracting the cradle into the main body of the tool which causes an upper end of the bit segment to slide along the head of the tool so as to extend laterally of the outer periphery of the tool. The bit locking sleeve is simultaneously pushed by the tool so as to catch the ends of and move inside the drill bit segments thereby expanding the drill bit segments to the inner diameter of the drive sub and locking the drill bit segments in the cutting position.
When lowering the tool into the ground drill the tool is initially placed within a transport sleeve which acts to compress the installation latch dogs to prevent catching on internal surfaces of the drill tube prior to entering a core barrel and the drive sub. A landing ring is provided between the core barrel and drill tube of a diameter which prevents further progress of the transport sleeve but allows the tool to pass therethrough. The transport sleeve sits on the landing ring and, after installation or retrieval of the cutting means again carries the tool once pulled from beneath the landing ring to the surface.
Field trials of the above system have proved very successful. Nevertheless, it is thought that there is a potential for various problems to arise under extreme operational conditions.
One potential problem with the above system of WO 94/29567 is that due to the relative lengths and configurations of the tool and the bit locking sleeve, when the tool bottoms out, that is, reaches the very end of its travel within the ground drill and stops, the bit locking sleeve may sit several millimetres above an abutment surface formed on the radially inner surface of the bit segments against which, ideally the bit locking sleeve should contact. When this contact is achieved, the maximum clamping force of the bit locking sleeve and bit segments against the inner surface of the drive sub is obtained. It is thought that there is a possibility of the bit locking sleeve effectively working its way loose under the influence of severe and sustained vibration when not disposed in direct contact with the abutment surface of the bit segments.